How did he hear about Tirhakah?
Malbim: By chance, he heard from passersby that Tirhakah left his land in order to fight him.
Why did he send to Chizkiyah again?
Rashi: He informed him that he is not totally leaving, rather, with intent to return [after he fights Kush].
Malbim: He knew that since he returns to Ashur, people will not rebel against Chizkiyah. He wanted that Chizkiyah will make Shalom with him and pay to him taxes and be his slave 1 . If not, it would be better not to inform that he will return, and Chizkiyah will cease to prepare for war!
Perhaps he was concerned for his honor. He did not want people to think that he threatens and does not carry out! (PF)
Chizkiyah did not answer Ravshake to seek Shalom even when Sancheriv was about to attack, before Chizkiyah prepared enough for war (refer to Melachim II, 18:20:1:5). Surely he will not seek Shalom now that Sancheriv goes away, and there is more time to prepare! (PF)
